During certain periods of differentiation, embryonic tissues become partially sensitive to the influence of factors in their environment. One recent example causing concern is the association of the appearance of a rare form of vaginal cancer in young women with ingestion of a hormone - the synthetic estrogen, diethylstilbestrol (DES) by their mothers during pregnancy. The period during pregnancy when the drug was taken seems to be critical in determining whether the female offspring will be affected; furthermore, the use of other drugs, such as progesterone, has not been correlated with the appearance of vaginal cancer. Therefore, there seems to be a period in the development of human vaginal tissue when the cells are exceptionally sensitive to a specific hormone. This project seeks to determine whether there is a period in the development of the female rat when mammary tissue can be affected by hormones such as DES. The effects of prenatal exposure to a hormone will be examined histologically at late fetal and neonatal stages to determine whether there are any immediate morphological changes in the immature mammary tissue. At 50 days of age, female rats (exposed to hormone in utero) will be treated with the carcinogen, 7,12 dimethylbenz(a)anthracene, to induce mammary tumors. The effect of a hormone will be measured as alterations in the incidence, growth characteristics and histological types of mammary tumors induced by the carcinogen. Alterations of these parameters of tumor induction will reflect a modified sensitivity to the action of the carcinogen within mammary tissue exposed to a hormone during fetal development. Demonstration of effects on mammary tumor incidence and growth after prenatal hormone exposure in rats could be a significant warning that an increased risk for development of breast cancer exists in young women whose mothers were given DES during pregnancy.